Single Phase Inverters on 208 3 Phase

If I lost power coming into the building, how will 6-7 banks of single phase inverters recreate the 3 phases? Could they be self starting and create the 3 phases correctly timed?
 
bellington said:
If I lost power coming into the building, how will 6-7 banks of single phase inverters recreate the 3 phases? Could they be self starting and create the 3 phases correctly timed?
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I think you’re thinking of a motor

They’ll just load each phase
 
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bellington said:
If I lost power coming into the building, how will 6-7 banks of single phase inverters recreate the 3 phases? Could they be self starting and create the 3 phases correctly timed?
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Grid following inverters can never do the above. PV inverters are often grid following only.

But if you have a single 3 phase inverter capable of running in grid-forming mode, e.g. the inverter attached to your BESS (likely but not definitive; what are the details on your battery system?), then all other grid following inverters will sync to it. Whether they are 3 phase or single phase.

Basically if some other feature of the PV system requires that you use single phase inverters (like using a solar shingle whose supplier will only supply single phase inverters), you can make it work fine. The downside will be 21-24 separate inverters and the associated wiring, rather than 2-4 large inverters. Just a logistical challenge.

Cheers, Wayne
 
Thanks. Battery system will be in another building with 3 phase inverter. If it failed, or was disconnected for some reason, then the 24 individual inverters cannot continue to operate in 3- phase mode, correct?
 
I do appreciate all the comments and the effort to help me grasp this complex situation. I understand the resultant waveform for the two A-N and N-B passing through a load to get A-B.

I don't understand how pushing the resultant backwards provides the same results in the two coils A-N, N-B. At time zero, the resultant voltage A-B is +147.224, A-N wants to be 0, and N-B is +147.224. How can A-N see anything but +147.224 volts and have electrons moving in the same direction and velocity as the resultant A-B? A-N cannot just stay at 0.

By 0.0027778 of a second, A-B is at peak +294.449, A-N wants to have reached +147.224 headed toward its +peak, but N-B wants to fall from +170 to +147.224, with electrons moving in the opposite direction of both A-B and A-N, but A-B is still pushing its electrons on the other direction.

Another 0.0013889 of a second and A-N wants to reach its +peak, but A-B and N-B are both decreasing. A-N cannot reach its +peak if A-B has already started falling.

So far, I can see how the formulas and graphs presented for the RESULTANT as seen by a load on A-B works. I can see how an inverter can send back the resultant waveform and provide the same power to an electric stove, or water heater. But, I cannot see that single waveform being effective to replicate those two waveforms going backwards into the two coils of the transformer.

Help!
 
bellington said:
So far, I can see how the formulas and graphs presented for the RESULTANT as seen by a load on A-B works. I can see how an inverter can send back the resultant waveform and provide the same power to an electric stove, or water heater. But, I cannot see that single waveform being effective to replicate those two waveforms going backwards into the two coils of the transformer.
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I don't know what else I can tell you; it works, and VA in = VA out. Does this help?

Ia = sqrt(Iab^2 + Ica^2 + (Iab)(Ica))
Ib = sqrt(Ibc^2 + Iab^2 + (Ibc)(Iab))
Ic = sqrt(Ica^2 + Ibc^2 + (Ica)(Ibc))
 
bellington said:
I do appreciate all the comments and the effort to help me grasp this complex situation. I understand the resultant waveform for the two A-N and N-B passing through a load to get A-B.

I don't understand how pushing the resultant backwards provides the same results in the two coils A-N, N-B.
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Simply, it doesn't. You need something else in the system to provide the neutral voltage reference. That something is the grid. (Or maybe it's a zig-zag transformer, but usually it's the grid.)

For example...
If your solar inverter is outputting 10A on A-B at 208V and you have two 10A loads each on A-N and B-N at 120V, the grid will supply the missing VA to the loads. This is because the grid is a voltage source and will work to maintain the A-N and B-N voltage waves. As you've noted, there are certain 30deg periods in the sine wave where the line-line inverter is pushing voltage and current in the wrong direction for a given line-neutral load. During those periods the grid will push enough voltage and current to the load in the other direction to counterbalance this. This is how a voltage source and a current source act when connected in parallel.

As ggunn has attested, he has installed this type of system (many of them, actually) and it works. The customer does not complain that they don't have 120V on their line-to-neutral loads.

There are power factor changes and the grid needs to have enough VA to supply the vars.
 
ggunn said:
I don't know what else I can tell you; it works, and VA in = VA out. Does this help?

Ia = sqrt(Iab^2 + Ica^2 + (Iab)(Ica))
Ib = sqrt(Ibc^2 + Iab^2 + (Ibc)(Iab))
Ic = sqrt(Ica^2 + Ibc^2 + (Ica)(Ibc))
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But you just told him the line currents and he's struggling to understand how the line-neutral voltage is maintained.
 
Ggunn basically he is asking how it's possible to backfeed a wye grid with a delta inverter. It's a legit theoretical question. We know it works. Can we explain exactly how? (Even if he doesn't need to understand it to know it will work.)
 
jaggedben said:
But you just told him the line currents and he's struggling to understand how the line-neutral voltage is maintained.
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OK, but L-L voltage is L-N voltage times sqrt(3), and the inverters match what they are connected to. Once again, I encourage the OP to study the fundamentals of three phase power.
 
jaggedben said:
Ggunn basically he is asking how it's possible to backfeed a wye grid with a delta inverter. We know it works. Can we explain exactly how? (Even if he doesn't need to understand it know it will work.)
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If the single phase inverters are connected line to line, it is essentially a delta to delta connection. Does that help or make it worse?

If the system is balanced, there is no current on the neutral; the neutral is only providing a reference for the phase voltages. If the PV system is not balanced, then what would be the neutral current in a wye is shared among the phase conductors, but the voltages are clamped by the service. The VA is different for each conductor because the current is different but the total VA out still equals the total VA in.

At the risk of adding confusion rather than reducing it, many (most?) three phase inverters do not require a neutral conductor, only a reference to ground, so they are effectively delta connected to a wye.
 
First, expressing all numbers as decimals hides their meaning. So I'm going to edit my quote of your post by replacing decimals with their relevant symbolic expressions. The sqrt(2) factor comes from the ratio of peak voltage to RMS voltage.

bellington said:
I don't understand how pushing the resultant backwards provides the same results in the two coils A-N, N-B. At time zero, the resultant voltage A-B is 120V*sqrt(2)*sin(120 deg), A-N wants to be 0, and N-B is 120V*sqrt(2)*sin(120 deg). How can A-N see anything but 120V*sqrt(2)*sin(120 deg) volts and have electrons moving in the same direction and velocity as the resultant A-B? A-N cannot just stay at 0.
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Yes, A-N can stay 0, and if A-B = N-B as you've correctly stated, it must be zero. When we define voltage between two points, the resulting values have to satisfy certain rules. The most fundamental is that adding up the voltages around a loop is always 0. So for voltages, A-B = N-B + A-N, always. When A-B = N-B, that leaves us with A-N =0.

I'm not sure how you conceive of voltage difference, but it's a difference of electrical potential energy. So an analogy for gravity near a small area of our planet's surface is just height difference. If the height difference from B to A equals the height difference from B to N, then A and N have to be at the same height.

[And just like we can define an absolute height if we pick a zero reference, say that mean sea level is height 0, the same works for voltage. Often we choose N as the zero voltage reference; then we can just talk about the voltage at A and at B, etc. But for the behavior between two other points in the circuit, like A and B, we still have to take the difference of those values to get the relevant voltages.]

Also, the voltage between two points is different from the current between two points. Just because one point has higher voltage than another doesn't mean that electrons are necessarily moving from the higher voltage to the lower voltage. That happens for a resistive load, but this discussion requires going beyond that simple case.

I didn't check the rest of your decimals to see if you have the correct values or not, as I think your first paragraph already provided enough for discussion.

bellington said:
So far, I can see how the formulas and graphs presented for the RESULTANT as seen by a load on A-B works. I can see how an inverter can send back the resultant waveform and provide the same power to an electric stove, or water heater. But, I cannot see that single waveform being effective to replicate those two waveforms going backwards into the two coils of the transformer.
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I think you may be misapprehending the difference between voltage and current. With a grid-following inverter, the voltages in the entire system are determine by the grid (or in a blackout, by a grid-forming inverter elsewhere in the system). All of your discussion earlier in your post was just about voltage waveforms, and how they are related A-B, N-B, and A-N.

The grid-following inverter can't change any of those voltages; all it can do is pump out current as required to transfer power. So the system voltages are all fixed, and we just need to think about the current. When the inverter is connected A-B, it pumps out current as the exact negative of what a water heater connected A-B would do. If there happens to also be a water heater connected A-B, the current the inverter pumps out can be thought of as going directly to that water heater, end of story (as long as the water heater's current demand is no smaller than the inverter's current output).

But in the case where there are no other loads between the inverter and the transformer, then the current passes back through the transformer. The relationship between the resulting primary side current and the secondary side current the inverter is creating is exactly the same as the relationship between the primary side current and the secondary side current for a water heater connected A-B, except for a minus sign in the case of the inverter.

In other words, the single phase grid-following inverter is just a "negative water heater".

Cheers, Wayne
 
wwhitney said:
The same way a wye grid can supply a delta connected load, just with a minus sign on the currents?

Cheers, Wayne
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I think his confusion is not understanding that the two sources remain present and connected in parallel at all times regardless of the net direction of power flow.

He is correct that if you have *only* a delta source present and feed it into the wye side of a typical wye-wye or wye-delta transformer, the transformer does not create a wye. (Only a special transformer like a zig-zag could do that, right?) He is imagining this to be the situation if power is flowing from delta-configured in inverters to the other side of the wye transformer. Or if it is trying to supply L-N loads all by itself.

What he's failing to understand is that the wye grid source remains present at all times when you are connecting a single phase or delta grid-tie inverter in parallel. Even if you're backfeeding the transformer in terms of net power flow.

Or to rephrase how I put it ggunn: can we explain how line-neutral *loads* still get the right voltage and current when you backfeed a wye system with a delta source?
 
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jaggedben said:
He is correct that if you have *only* a delta source present and feed it into the wye side of a typical wye-wye or wye-delta transformer, the transformer does not create a wye. (Only a special transformer like a zig-zag could do that, right?)
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Pretty sure that if you have a delta source and connect it to the wye side of a transformer, the center connection point of the wye will become a neutral point of your delta, and you can derive a neutral that way. [Edit: maybe that's only for the wye-delta case. This requires some more thought.] Not clear on what advantage the zig-zag has for this application; perhaps there is some serious problem with deriving a neutral this way.

jaggedben said:
Or to rephrase how I put it ggunn: can we explain how line-neutral *loads* still get the right voltage and current when you backfeed a wye system with a delta source?
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Well, the voltages in the system are set by the primary source, the backfeeding grid-following inverter doesn't change any of the voltages. So the L-N loads see the same voltages with or without the backfeed.

As to the currents, you can consider the two cases separately: load only on the primary source, or backfeed only on the primary source. The combined loading condition will have currents that are just the sum of those two cases.

Or putting it another way, the current the load wants may be considered to be satisfied by the backfeeding inverter to the extent that it provides the correct current component, with the remainder of the backfed current going further towards the primary source, and with any shortfall made up by the primary source.

Cheers, Wayne
 
wwhitney said:
Pretty sure that if you have a delta source and connect it to the wye side of a transformer, the center connection point of the wye will become a neutral point of your delta, and you can derive a neutral that way. [Edit: maybe that's only for the wye-delta case. This requires some more thought.] ...
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Well I could stand to be further educated on this. But I think we agree that you don't get the neutral point if you have only one single phase inverter feeding that transformer with no other source present. (Or two on different phases, for that matter.) He keeps presenting us with that scenario, and we keep having to say that won't be all that's going on.
 
wwhitney said:
Pretty sure that if you have a delta source and connect it to the wye side of a transformer, the center connection point of the wye will become a neutral point of your delta, and you can derive a neutral that way. [Edit: maybe that's only for the wye-delta case.
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I'm not sure precisely what you are driving at, but if you connect a delta source, e.g., an inverter without a neutral, to a a wye service or transformer secondary with a grounded neutral, the neutral provides the zero V reference for the phase voltages, and the inverter needs to reference it as well, but obviously a delta source has no neutral. In practice a three phase inverter can either have a neutral connection back to the service or transformer, or there can be a grounded "neutral" at the inverter, but obviously not both; running a neutral to a neutral agnostic inverter would be a waste of money, so I don't do it unless a customer wants it for some reason.
 
ggunn said:
I'm not sure precisely what you are driving at
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This is an aside from the OP's scenario: say you have an (ungrounded for simplicity) 480V delta 3W service from the utility, and you have a transformer with a 480Y/277V primary, so you hook up your 480V 3W delta to H1-H3, and leave the secondary disconnected. Can we use H0 as a neutral point and power a 277V load using it? And does the answer depend on whether the secondary is a wye or delta?

Something I need to think about, but not relevant for the OP.

Cheers, Wayne
 
jaggedben said:
But I think we agree that you don't get the neutral point if you have only one single phase inverter feeding that transformer with no other source present. (Or two on different phases, for that matter.)
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I think that if you had two single phase grid-forming inverters connected A-B and B-C, and they had the firmware and connection to talk to each other and sync their voltage waveforms to be 120 degree apart, effectively acting as an open delta source, then depending on the answer to the previous question, you could get a neutral point.

But for one or two non-communicating single phase grid-forming inverters, definitely not.
jaggedben said:
He keeps presenting us with that scenario, and we keep having to say that won't be all that's going on.
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I don't think that the scenario is so precisely defined in the OP's posts. My latest read is that the OP thinks the single phase inverter has to somehow recreate the voltage waveforms A-N and N-B all on its own, which is never true for a grid-following inverter.

Cheers, Wayne
 
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